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Spectrokinetics study of probable effects of diverse inorganic ions on bleaching of dye

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Toluidine blue (TB) is an important anticoagulant metachromasia molecule showing a pronounced variation in the visible spectrum due to the aggregation phenomenon and electrostatic interaction with the charged synthetic and biologic polymers. The
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  RESEARCH ARTICLE Spectrokinetics study of probable effects of diverse inorganicions on bleaching of dye Ra 󿬁 a AZMAT( ✉ ) 1 , Masooda QADRI 2 , Fahim UDDIN 2 1 Department of Chemistry, Jinnah University for Women, 5C Nazimabad, Karachi 74600, Pakistan2 Department of Chemistry, University of Karachi, Karachi 75270, Pakistan © Higher Education Press and Springer-Verlag Berlin Heidelberg 2010 Abstract  Toluidine blue (TB) is an important antic-oagulant metachromasia molecule showing a pronouncedvariation in the visible spectrum due to the aggregation phenomenon and electrostatic interaction with the chargedsynthetic and biologic polymers. The current studydescribes the interactive role of diverse inorganic materialions on the bleaching of toluidine blue (tolonium chloride)(TB + ) with urea in acidic and basic media using thespectrophotometric technique. The spectra of TB and ureawith different cations and anions were monitored and their characteristic features are presented here. The negativeeffect of added cations on reduction may be the result of altered electron pathways which led to suppression of thereduction/bleaching of TB, while a slight decrease in dyereduction by added anions may be due to the scavenging of the OH* radical. It has been observed in the case of Co 2 + that in addition to the electron-transfer reaction, other  processes like layer and precipitate formation also appear to be taking place. The dye bleaching process followed pseudo  󿬁 rst order kinetics with respect to TB, urea, and H + ion, whereas signi 󿬁 cant decoloration in the presence of urea proved that reductants control the redox reaction. Nodecoloration in acidic medium with diverse ions was seencompared to alkaline media, showing that water pH playedan important role in the bleaching of dye. The reduction/  bleaching of dye was investigated at different tempera-tures, and energy parameters were evaluated for a TB + -Urea reaction, including the energy of activation (  E  a   =39.60 kJ $ mol  –  1 ), enthalpy of activation ( ∆  H  # =34 kJ $ mol  –  1 ), entropy of activation ( ∆ S  # =146.5kJ mol  –  1 $ K   –  1 ), and free energy of activation ( Δ G* =  –  52.35kJ $ mol  –  1 ). A mechanism of interaction of diverseions in dye bleaching and a mechanism of reduction basedon the above  󿬁 ndings is proposed. Keywords  TB, diverse ions, suppress, decoloration 1 Introduction Dyes have been in use for several years due to their superior performance in a variety of products including paper and pulp, adhesives, art supplies, beverages,ceramics, construction, cosmetics, food, glass, paints, polymers, soap, wax biomedicine, etc. However, dyewaste water is rated as the most hazardous material,containing a complex and highly variable mixture of manyother substances ranging from inorganic compounds andelements to polymer and organic compounds [1]. The presence of residual color, high levels of electrolytes, andtoxic substances (e.g. metals and un-reacted raw materials)in the dye application process produces waste water that  poses unacceptable environmental risks [2]. In the case of textile dying operations, the concern can arise fromincomplete dye bath exhaustion and the presence of dyeauxiliaries and metals ions that are toxic to aquatic life andreduces light penetration and photosynthesis [3,4]. The discharge of heavy metals into the aquatic ecosystemincreases the alkalinity of water. The mineral material,mostly sodium salts, increase the salinity of water. One of the most serious environmental problems in the dye,textile, and leather industries is associated with themanufacture and use of metalized dyes that are complexedwith Cr and Co to obtain desirable fastness [5]. Commonly,urea is used to maintain the swelling of the  󿬁  ber whichkeeps the dye in solution and increases the diffusion timeduring which the dye is delivered to the interior   󿬁  bers.Yeung and Shang [6] observed that the addition of metalions to dye solutions results in promoting dye aggregationwith the promotingeffects of calcium andmagnesium ions,which is greater than that of sodium ions under acidicconditions; a higher concentration of metal ions wouldresult in an increase in dye aggregation with a reduction in Received April 28, 2010; accepted August 22, 2010E-mail: ra 󿬁 asaeed200@yahoo.com Front. Chem. Sci. Eng. 2011, 5(1): 131  –  138DOI 10.1007/s11705-010-0556-z  dye hydrophobicity. On the other hand, dye aggregation isincreased in alkaline conditions under the in 󿬂 uence of metal ions; hydrophobicity has been shown to increase [7  –  9]. Chen et al. [10] studied the effect of metal ions (Cu 2 + ,Fe 3 + , Zn 2 + , Al 3 + , Cd 2 + ) on the photodegradation of severaldyes such as sulfo-rhodamine B (SRB), alizarin red (AR),and malachite green (MG) in aqueous TiO 2  dispersionsunder visible irradiation ( l >  420nm).The aim and objective of fresh research is to focusattention on dye waste water containing urea and other toxic metals ions which are commonly used in the dyeapplication process. This study covers the interactive roleof untreated dye and urea with diverse ions in the wastewater, using UV-Visible radiation on a spectrophotometer.For this purpose TB was selected due to its wider use in thedifferent   󿬁 elds of medicine and surgery, biological cellstaining and printing, etc. [11  –  13]. The article covers thekinetics and mechanism of reactions of TB with urea inacidic and alkaline media with probable interactions of themost common anions and cations in the dye waste water. 2 Materials and methods The experiment is divided into four sections: i) preparationof solutions, ii) kinetic measurements, iii) data analysis,and iv) spectral analysis of the dye interaction withinorganic material ions. 2.1 Preparation of solutions Stock solutions of toluidine blue (TB + ) = 1.0   10  –  4 mol $ L  –  1 , urea = 0.5 mol $ L  –  1 , HCl = 0.5 mol $ L  –  1 , NaOH = 0.50 mol $ L  –  1 , and NaCl = 0.60 mol $ L  –  1 , Na 2 SO 4  = 0.5mol $ L  –  1 , NaNO 3 =0.5mol $ L  –  1 , CoCl 3  =0.5mol $ L  –  1 , MgCl 2  = 0.5mol $ L  –  1 , Na 2 CO 3  = 0.5mol $ L  –  1 ,and NaHCO 3  = 0.5mol $ L  –  1 in 250mL were prepared indeionized water. Necessary dilutions were made at the timeof kinetics measurements. 2.2 Kinetic measurements Initially, each kinetic run was made by keeping onereactant varied and the others taken as constant. Kineticruns were pursued by measuring the absorbance of amixture of thermostatic solutions as a function of time at regular 60s intervals up to 30min [14,15]. The absorption spectrum of TB was scanned for absorption maximum peak at a wavelength of 596nm ( l max ). The requisite ionicstrength of the medium was maintained by addingappropriate volumes of NaNO 3  = 0.5mol $ L  –  1 solution[14]. 2.3 Data analysis According to the Beer-Lambert law (  A  =  ε  bc ), theabsorbance  A  of a dilute solution is proportional to itsconcentration  c , and path length  b . Under the experimentalconditions, the TB + concentration obeys the Beer-Lambert law. A plot of log [TB + ]  vs  time or log  A vs  time should belinear with a slope  k  /2.303. Here,  k   is a pseudo  󿬁 rst order rate constant. Under pseudo- 󿬁 rst-order conditions, a linear  plot of log  A  or log Δ  Avs  time indicates the  󿬁 rst-order dependence of the rate on TB + , and the slope of the log[urea]  vs  log k   plot will be equal to the order of the reactionwith respect to urea [15,16]. 2.4 Spectral analysis of the dye interaction with inorganicmaterial ions Thereaction kinetics of TBwas studied with varying initialconcentrations of anions and cations which are commonly present in dye waste water solution. Spectra were scannedin both acidic and alkaline media to check the effect of these ions on bleaching of the dye with urea. 3 Results and discussion The initial reduction rate and the extent of decoloration arereported in Tables 1  –  3. The time dependence of the opticaldensity is linear and the slope of the line is equal to the rateconstant for different initial concentrations of dye, urea,and H + ion. The kinetics of dye reduction with respect toits change in optical density 󿬁 tted well into a 󿬁 rst order rateequation, ln ð  A o Þ  –  ln ð  A t  Þ ¼  kt  , (1)where  k   is constant and  A  is the optical density at initial andin 󿬁 nite time, respectively. The pseudo  󿬁 rst order ratecoef  󿬁 cients ( k  ) were obtained using the linear regressionmethod. Table 1 shows the rate constant data obtained for varied initial concentrations of urea with  󿬁 xed concentra-tions of HCl and the dye [14  –  19], and a representativegraph is shownin Fig. 1. Itcan be observed fromFig. 1that the dye decoloration is directly related with the ureaconcentration. However, at high concentrations of urea it isnot linear, which may be due to the de-excitation of dyemolecules according to following mechanism,TB  ↕       ↓ TB  þ  h v  :  (2)The kinetics of the redox reaction of TB and ureasuggest that the reduction depends on the concentration of urea as a reductant and the activated state of the dyemolecule [15], and no change was observed without urea,showing that a reductant is essential for the oxidationreduction reaction of dye. Therefore, the rate of reactionmay be as shown in Eq. (3) [19]:  –   d ½ TB þ  d t   ¼  k  ½ TB þ ½ urea  :  (3) 132 Front. Chem. Sci. Eng. 2011, 5(1): 131  –  138  The ln[urea]  vs  ln k   is a straight line passing through thesrcin, suggesting second order kinetics with respect to thereductant with a rate constant   k  ’  = 9.2    10  –  3 mol.L  –  1 [Fig. 2]. Kinetics of the dye reduction were measured byvarying the concentration of an acid and the dye, with 󿬁 xedconcentrations of the other two remaining variables [15].The results are summarized in Tables 2 and 3. Table 2shows the dependence of reduction on H + ion concentra-tion. The plot of [H + ] with  k  ’  was a straight line, indicating pseudo  󿬁 rst order rate reliance (Fig. 3). The varyingconcentration of the dye (Table 3) in the reaction mixturesuggests that initially, the rate constant   “ k  ”  decreases but at higher concentrations it becomes constant, indicating that the dye reduction process was independent of the dyeconcentration (Fig. 4).The acidity of the reaction mixture was varied bychanging the concentration of hydrochloric acid andsodium hydroxide; the pH of the reaction mixture wasrecorded using a pH meter. Results showed that in the presence of an acid and urea with a low concentration of  Fig. 1  Effect of urea on reduction of dye at temperature = 25°C,[TB] = 2.0    10  –  5 mol $ L  –  1 , [H + ] = 1.0    10  –  1 mol $ L  –  1 Table 1  In 󿬂 uence of urea concentration on rate and % decoloration of TB  a) [urea] /( mol $ L  –  1 ) d  x d t     10 4 =  mol $ L  –  1 $ s  –  1    k     10 2 /s  –  1 decoloration/  % 0.05 2.0 1.29 69.11.00 2.0 1.32 721.5 3.0 1.34 752.0 4.0 1.54 77.52.5 4.0 1.41 78 a) Temperature = 25°C, [TB] = 2.0    10  –  5 mol $ L  –  1 , [H + ] = 1.0    10  –  1 mol $ L  –  1 Table 2  In 󿬂 uence of H + concentration on rate and % decoloration of TB  a) [H + ] /( mol $ L  –  1 ) d  x d t     10 4 =  mol $ L  –  1 $ s  –  1    k     10 2 /s  –  1 decoloration/  % 0.03 1.0 1.7 650.06 2.0 2.8 700.1 3.0 2.9 720.2 4.0 4.0 780.4 8.0 6.7 80 a) Temperature = 25°C, [TB] = 2.0    10  –  5 mol $ L  –  1 , [urea] = 1.0    10  –  1 mol $ L  –  1 Table 3  In 󿬂 uence of TB on rate and % decoloration  a) [TB]    10 5 /( mol $ L  –  1 ) d  x d t     10 4 =  mol $ L  –  1 $ s  –  1    k     10 2 /s  –  1 decoloration/  % 1.00 4.00 1.61 682.00 3.00 1.34 603.00 3.00 1.27 504.00 3.00 1.27 505.00 3.00 1.27 50 a) Temperature = 25°C, [urea] = 1.0    10  –  1 mol $ L  –  1 , [H + ] = 1.0    10  –  1 mol $ L  –  1 Fig. 2  The plot of ln  k vs  ln [urea] Ra 󿬁 a AZMAT et al. Spectrokinetics of effects of diverse inorganic ions on bleaching of dye 133  the dye, the reduction reaction followed pseudo  󿬁 rst order kinetics that signify pseudo  󿬁 rst order dependence withrespect to the H + ion, reductant and dye(Table 2). Thespectral scan showed that bleaching takes place morerapidly at alkaline pH than in acidic pH, without a changein the  l max . This may be attributed to the strong oxidizing power of the hydroxyl radical. The hydroxyl radicals react with the dye axochrome groups which commonly controlthe color of the dye or show af  󿬁 nity towards the CH 3 radical, resulting in demethylation with the production of the intermediate cause of the bleaching of the dye [15].This may be the probable source of the initiation of dyedecoloration at alkaline pH [20,21]. The change in absorption values as a measure of  bleaching of the dye used to evaluate the percentagedecoloration (Table1) and representative plots are shown inFigs. 1 and 4. The plots of   %  decoloration  vs  reductant (Fig. 1) showed the signi 󿬁 cance of the reductant for theredox reaction of dye reduction, while depletion indecoloration with increasing concentration of dyeoccurred, re 󿬂 ecting the negative relation with the number of photons absorbed by the increasing number of dyemolecules (Fig. 4) [19  –  22]. 3.1 Effects of added cations on dye reduction The reduction of dye was investigated with some essentialand trace metal cations (like Na + , K  + , Mg 2 + , and Co 2 + ) toverify whether these metals have some effects on dyedecoloration. The results are summarized in Table 4. Thedetrimental effects of cations were measured by varyingthe concentration of these ions at an alkaline and an acidic pH. No change in dye reduction was observed at low pH,whereas a high pH showed the aggregation of dyemolecules with reduction. When trace quantities of theseions were added in the dye reaction mixture, it wasobserved that these cations showed censored effects on dyereduction, with completely different behaviors of thetransition metal Co 2 + and alkaline earth metal Mg 2 + , whereinstead of the bleaching of dye, precipitate formation in thecase of Mg 2 + and both precipitate and layers formation inthe case of Co 2 + were observed. Co 2 + showed a completelydifferent behavior when compared to other ions. This may be related to the modi 󿬁 ed electron-transfer pathway whichis involved in the reduction of the dye that markedlydepresses the photo reduction of TB + under visibleradiation. The presence of these metal ions may result in polymerization or aggregation or complex formation or adsorption of the dye on the metal surface. Scan spectra of the dye in the presence of Co 2 + is shown in Fig. 5, whichre 󿬂 ects the unusual behavior of the dye [10,20  –  22] withthis metal.The precipitate formation in the presence of Co 2 + andMg 2 + may obstruct the photons of the light, due to whichno decoloration of dye [14] is observed (Table 4), which Fig. 3  The plot of   k   vs H + for reduction of TB  vs  urea Fig. 4  Effect of dye on reduction of dye. Temperature = 25°C,[urea] = 1.0    10  –  1 mol $ L  –  1 , [H + ] = 1.0    10  –  1 mol $ L  –  1 Table 4  Effect of different ions on TB + -urea reaction  a) ions  k     10 2 /s  –  1 decoloration/  %  literature [9] comparison Na +  –   2.0  –  K  +  –   2.0  –  Mg 2 +  – – –  Co 2 +  – – –  CO  –  3  8.9 14.59 23 H CO  –  3  1.18 12.90 23 S O  –  4  9.15 36.69 38 N O  –  3  8.11 15.81 20Cl  –  1.15 16.00 16.08 a) Temperature = 25°C, [TB + ] = 2.0    10  –  5 mol $ L  –  1 , [NaOH] = 1.0    10  –  1 mol $ L  –  1 , urea = 1.0    10  –  1 mol $ L  –  1 , time of irradiation = 1200 s 134 Front. Chem. Sci. Eng. 2011, 5(1): 131  –  138  may result in the complex formation seen in Eqs. (4) and(5).TB þ þ  Co 2 þ þ  NH 2   CO    NH 2 ↕       ↓ Co TB þ ð Þ  NH   CO    NH 2 ½  complex ð Þ 2 þ (4)TB þ þ  Mg 2 þ þ  NH 2   CO    NH 2 ↕       ↓ Mg TB þ ð Þ  NH 2   CO    NH 2 ½  complex ð Þ 2 þ (5)Other metal ions such as Na + and K  + have only a slight in 󿬂 uence on the photo reduction [23] of TB as compared toMg 2 + andCo 2 + , which may be related to the oxidation stateof these metal ions (Na + = K  + >  Mg 2 + = Co 2 + ). Otherwise,it would be safe to analyze the effect of these cations on the basis of valence state, where the valence of cations hassome bearing on the reduction of TB with urea, due towhich depletion in the bleaching of dye is observed asreported in Table 4. This results in the modi 󿬁 ed electron pathway leading to the semi leuco dye formation in the presence of Na and K according to Eq. (6). 3.2 Effect of Added anions on decoloration kinetics The dye reduction was also studied in the presence of anions such as SO 2  –  4  , HCO  –  3  , CO 2  –  3  , Cl  –  , N O  –  3  within therange of low and high pH. Theses ions were selected because of their presence in the dye waste water. It was observed that the dyereduction proceeds more rapidlyat alkaline pH compared to that at low pH whichshows that dye reduction is affected by the pH of themedium. Results are reported in Table 4 which shows that dye reduction/decoloration was signi 󿬁 cantly higher in presence of sulfate ions as compared to other anions. Thisis probably due to the reaction of SO 2  –  4  with OH  –  asreported earlier [24], through which an excited species of sulfate radical is formed [7  –  9], or it may be due to twoelectrons of SO 2  –  4  (Eq. (7)) which can play a considerablerole in dye reduction according to followingmechanism:SO 2  –  4  þ  OH   ↔      SO 2  –  4  þ OH  –  (7)All added anions get excited according to the abovereaction [7  –  9] and enter in a reduction reaction throughde-excitation by releasing a photon during collision withthe dye molecule resulting in the formation of a semireduced dye according to Eq. (6), which later on isconverted into a leuco dye by H abstraction from thereductant urea.CO 2  –  3  þ  OH   ↔      CO 2  –  3  þ OH  –  (8)HCO  –  3  þ  OH   ↔      HCO  –  3  þ OH  –  (9) NO  –  3  þ  OH   ↔      NO  –  3  þ OH  –  (10)Cl  –  þ OH   ↔      HO  Cl  –  þ  OH  –  (11)A comparison of the percent decoloration presented inTable 4 with the percent decoloration of other variablesreported in the Tables 1  –  3 shows that the percent decoloration decreases in the presence of added anions,which may be due to the scavenging of excited OH*radicals [7  –  9] which then leads to a suppressive effect onthe reduction of dye (Eq. (9  –  11)). The spectra of dye in the presence of anions at alkaline pH re 󿬂 ects the role of anionsin the dye reduction reaction with urea and shows that thedye reduction takes place variably in the presence of theseions without a shift in the wavelength, con 󿬁 rming theformation of leuco dye (MGH) according to Eq. (12) andFig. 6. A comparison of the literature supports themechanism of reduced reduction in the presence of added anions [9]. Fig. 5  Absorption spectrum of TB  vs  urea in presence of Co Ra 󿬁 a AZMAT et al. Spectrokinetics of effects of diverse inorganic ions on bleaching of dye 135
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